Apolipoprotein B-related gene expression and ultrastructural characteristics of lipoprotein secretion in mouse yolk sac during embryonic development.

In mice, the yolk sac appears to play a crucial role in nourishing the developing embryo, especially during embryonic days (E) 7;-10. Lipoprotein synthesis and secretion may be essential for this function: embryos lacking apolipoprotein (apo) B or microsomal triglyceride transfer protein (MTP), both of which participate in the assembly of triglyceride-rich lipoproteins, are apparently defective in their ability to export lipoproteins from yolk sac endoderm cells and die during mid-gestation. We therefore analyzed the embryonic expression of apoB, MTP, and alpha-tocopherol transfer protein (alpha-TTP), which have been associated with the assembly and secretion of apoB-containing lipoproteins in the adult liver, at different developmental time points. MTP expression or activity was found in the yolk sac and fetal liver, and low levels of activity were detected in E18.5 placentas. alpha-TTP mRNA and protein were detectable in the fetal liver, but not in the yolk sac or placenta. Ultrastructural analysis of yolk sac visceral endoderm cells demonstrated nascent VLDL within the luminal spaces of the rough endoplasmic reticulum and Golgi apparatus at E7.5 and E8.5. The particles were reduced in diameter at E13.5 and reduced in number at E18.5;-19. The data support the hypothesis that the yolk sac plays a vital role in providing lipids and lipid-soluble nutrients to embryos during the early phases (E7;-10) of mouse development. secretion in mouse yolk sac during embryonic development.

An MTP inhibitor that normalizes atherogenic lipoprotein levels in WHHL rabbits.

Patients with abetalipoproteinemia, a disease caused by defects in the microsomal triglyceride transfer protein (MTP), do not produce apolipoprotein B-containing lipoproteins. It was hypothesized that small molecule inhibitors of MTP would prevent the assembly and secretion of these atherogenic lipoproteins. To test this hypothesis, two compounds identified in a high-throughput screen for MTP inhibitors were used to direct the synthesis of a highly potent MTP inhibitor. This molecule (compound 9) inhibited the production of lipoprotein particles in rodent models and normalized plasma lipoprotein levels in Watanabe-heritable hyperlipidemic (WHHL) rabbits, which are a model for human homozygous familial hypercholesterolemia. These results suggest that compound 9, or derivatives thereof, has potential applications for the therapeutic lowering of atherogenic lipoprotein levels in humans.

Cloning and regulation of hamster microsomal triglyceride transfer protein. The regulation is independent from that of other hepatic and intestinal proteins which participate in the transport of fatty acids and triglycerides.

Microsomal triglyceride transfer protein (MTP) is a heterodimer consisting of protein disulfide isomerase and a unique large subunit. Recent studies showing that an absence of MTP is a cause of abetalipoproteinemia indicate that MTP is required for the assembly of very low density lipoproteins in the liver and chylomicrons in the intestine. In this study, complementary DNA encoding the large subunit of hamster MTP was cloned. The cDNA sequence was used to design a 50-base pair oligonucleotide probe for a solution hybridization assay to quantitate MTP large subunit mRNA levels in a study of MTP regulation in male Syrian Golden hamsters. In animals fed a low fat diet, MTP exhibited a proximal to distal gradient of expression in the intestine. MTP activity and large subunit mRNA levels in the liver were about 25 and 10% that found in the proximal intestine, respectively. To investigate the effect of diet on MTP, hamsters were maintained for 31 days on one of four diets: 1) control low fat, 2) high fat, 3) low fat, high sucrose, or 4) diet 1 followed by a 48-h fast. The high fat diet increased MTP large subunit mRNA levels in the liver and throughout the small and large intestine. A 55 and 126% increase was observed in the liver and intestine (duodenum and jejunum), respectively. A 40% increase of intestinal MTP protein mass was also observed. The high sucrose diet caused a significant 55% increase in hepatic MTP mRNA levels but did not significantly affect the intestinal mRNA levels. MTP mRNA levels were unchanged in response to fasting. A short term dietary study showed that intestinal MTP mRNA was up-regulated within 24 h after initiating a high fat diet. An acute hepatic response was not observed. The regulation of MTP mRNA levels by high fat diets was compared to that of the liver fatty acid binding protein (L-FABP) and apolipoprotein B (apoB). ApoB mRNA levels were not significantly affected by a high fat diet. Although L-FABP mRNA levels were increased in the liver and intestine, the onset of the changes did not parallel that of MTP. These results suggest that L-FABP, apoB, and MTP, three proteins which play important roles in the transport of fatty acids and triglyceride in the liver and intestine, are not coordinately regulated by diet in hamsters.